Genetic studies on the transmission of murine retroviruses through the mouse germline, susceptibility to exogenous infection, and virus-induced oncogenesis have led to the identification and chromosomal mapping of numerous genetic loci involved in these phenomena. These studies rely on the analysis of somatic cell hybrids either alone or in conjunction with classical Mendelian crosses. The genes under investigation include proviral integration sites for various mammary tumor viruses and MuLVs, the cellular homologs of several oncogenes, and tumor-associated integration sites. Hybrids have also been used to map several other genes for which cloned probes are available including interferon and Alpha-, Beta-spectrin.